This invention relates to a method of butt-welding metal plates having a relatively narrow groove therebetween, and a welding apparatus for practicing the method, and to an improvement of a plasma arc generating torch employed in the apparatus.
For butt-welding steel plates of a large structure on a spot by a vertical or horizontal welding method, electro-slag welding and an electro-gas welding are popularly employed. The latter have various merits; for instance, the working of a welding groove can be readily achieved, and deformation due to welding is substantially absent. However, welding of to the base metal and therefore the portions of the welded plates affected by the heat are reduced, that is, the mechanical properties of such portions deteriorate.
More specifically, in the conventional electro-slag welding process or the conventional electro-gas welding process, the welding is conducted by maintaining the melting of the metal plates or the work, and the melting or a welding wire by feeding electric current between the welding wire and the work; therefore a groove provided between the metal plates should have a relatively large width and the welding wire also should have a relatively large diameter so that the melting rate (that is, the welding rate) of the welding wire is not greater than the melting rate of the plates; that is, the welding must be conducted at a low rate so that the metal plates are sufficiently molten.
The fact that the welding groove has a large width is convenient for inserting the welding wire thereinto, but is undesirable for the following reasons: the large width of the groove makes the welding rate slow, which leads to the application of excessive heat to the work. As a result, metal is excessively molten from the work, and the metallurgical properties of the weld of the work deteriorate. Thus, the conventional electro-slag or electro-gas welding method cannot be employed for welding materials such as thick plates, high-tension steel plates, or alloy steel plates which are greatly affected by heat. Especially in the electro-slag welding process, which is a kind of resistance welding process utilizing molten slag, the weld penetration of metal plates is insufficient at the start of the welding, and lack of fusion is produced, so that the work must be subjected to rewelding, or repaired by another welding method.
In order to overcome the above-described difficulties, recently a narrow gap welding method, i.e., one of several metal shield gas welding methods has been proposed. In this method, a narrow welding groove and a small welding current are employed to reduce heat applied to metal plates to be welded. As a result, the metallurgical properties of the work are improved. However, in this method, it is necessary to oscillate a welding arc by an oscillating mechanism and also to conduct multiple-layer welding. Furthermore, it is impossible to increase the welding rate because of the employment of a small welding current. This is, a specific feature of the narrow gap welding method, i.e., the reduction of heat applied to the work results in the lowering of its welding efficiency, and therefore the application of the narrow gap welding method is limited.
In order to overcome the above-described difficulties accompanying these conventional welding methods, applicant has proposed a vertical position welding method and a welding apparatus for practicing the method. However, this method is limited to the butt-welding of plates in a vertical position, and a welding wire is molten by an arc produced between the welding wire and a nonconsumable electrode torch, that is, a plasma arc generating torch, thus damaging the latter.
Thus, it is necessary to provide a novel welding method and a welding apparatus for practicing the method by which a weld high in quality can be achieved with high efficiency.
This requirement has led to the need to improve a plasma arc generating torch employed in the novel welding method due to the following reasons:
The specific features of a plasma arc generating torch are to compress the arc by a Lorentz's force which is directed radially to the center of a section of the arc by the action of a magnetic field generated by the arc current, and to improve the concentration factor of energy of the arc by restraining the arc column by the use of a thermal pinch effect which is obtained by cooling the outside surface of the arc column with a shielding gas and a plasma gas.
Accordingly, the electrode of an ordinary plasma arc generating torch is a cylindrical rod whose end portion is sharpened, so as to effect the energy concentration of the arc and also the stability of the same. However, in practice, the arc generated from the tip end of the sharpened electrode diverges greatly after it has passed its restraining nozzle, while a welding current allowable per welding electrode is limited. Accordingly, in order to increase the welding current or to apply a large welding current to the work, it is necessary to use a plasma arc generating torch having a plurality of electrodes or a large current capacity.
In addition to the fundamental plasma arc method described above, a magnetic control method and a gas discharge direction regulating method are known in the art for increasing the concentration of the energy of an arc. In the magnetic control method, a magnetic field is applied externally to an arc which has passed through an opening in its nozzle to obtain a flat arc column and thereby to increase the current density and the potential gradient of the arc. In the gas discharge direction regulating method, the nozzle has an elliptical or rectangular opening to control the discharge rate and direction of the gas so as to flatten the arc, as a result of which the same above-described effect is obtained.
In either of these two methods, the cylindrical arc produced by the sharpened electrode is restrained or deformed into a flat arc. Therefore, in the arc generated by the sharpened electrode, the current density is higher at the central portion of the arc column, that is, it is difficult to disperse the current in the direction of the major axis of the ellipse. In addition, in order to deform the arc only by means of the opening in the nozzle, it is necessary to feed a large quantity of controlled gas flow, which is liable to damage the nozzle.